This invention relates to a magnetically floating actuator having an angular positioning function which enables a rotor to be angularly positioned at will and which makes it possible to float the rotor magnetically and support it on a stationary member in a contactless manner.
In recent years demand has grown for an actuator suitable for use in a strictly controlled environment (e.g., an environment that demands a dust-free, noise-free, vibration-free or high-vacuum condition) and having a highly accurate angular positioning function.
More specifically, in a semiconductor manufacturing process, by way of example, fine machining on the order of microns to submicrons is required, so that the occurrence of dust is a significant problem since dust particles can have a major influence upon the semiconductor manufacturing yield. This makes it absolutely essential to prevent dust from being produced by the rotating parts of machinery. With conventional motors, however, the state of the art is such that the scattering of grease and minute particles produced by metal wear occurring at bearings cannot be avoided.
The above requirement is not limited to the field of semiconductor manufacture but also holds for various positioning mechanisms, machine tools, automatic machinery, conveyance machinery and robots that find use in vacuum, underwater or in extremely hostile environments, namely at locations or in environments where ordinary roller bearings, sliding bearings and fluid bearings cannot be employed.
An actuator of the above-described type not only should be mechanically contactless but preferably has a control system which is as simple as possible, such as a control system which does not require the provision of a rotational angle sensor for positioning.
What is first called to mind from the existing art under these circumstances is a magnetic bearing.
Conventionally, a magnetic bearing is used for supporting a rotary body in a contactless state and obtains the rotary body supporting force by utilizing magnetic attraction or repulsion.
FIG. 1 is a sectional view illustrating a motor of the type that uses such a magnetic bearing. As illustrated, a high-frequency motor 3 having a magnetic bearing is mounted on a base 1 through the intermediary of a vibration absorbing buffer 2. The magnetic bearing is composed of radial bearing units 4, 4' provided on either side of the high-frequency motor 3, and a thrust bearing unit 5 arranged at one end of a rotor 6. These units 4, 4', 5 are magnetically floated by electromagnets. An emergency bearing 10 is provided between the rotor 6 and an outer frame 9.
Displacement of the rotor 6 in the radial direction is sensed by a radial sensor 7, and displacement of the rotor 6 in the thrust direction is sensed by a thrust sensor 8. The position of the rotor 6 is controlled to assume a prescribed position by adjusting the attracting force of the electromagnet associated with the magnetic bearing.
Thus, in order to support the rotor 6 in a perfectly contactless state in the conventional magnetic bearing, it is necessary to control displacement or inclination in at least one degree of freedom. In other words, it is necessary to sense the gap between the rotor and a stator and control the attracting force of the electromagnet so as to hold the gap constant.
Meanwhile, control of the rotational motion of the rotor 6 is performed entirely independently of the magnetic bearing mechanism for contactless support. For example, there are many cases where the rotor is driven by the motor in the manner illustrated in FIG. 1, and the motor usually is an induction-type high-frequency motor. There are also cases where the rotational power is obtained by transmitting rotation, which is acquired externally via a joint, by means of a coupling. In either case, the magnetic bearing mechanism and the rotary drive mechanism are separable and are merely mechanically integrated. In other words, though the magnetic bearing section and the rotary drive section are integrated in mechanical terms, they clearly can be separated from each other. In addition, in order to drive the rotor 6 supported by the pair of radial magnetic bearings 4, 4', an induction-type high-frequency motor is almost always used. This motor is employed for effecting rotation at a high or constant velocity. In short, control of the rotor is exclusively for controlling rotational velocity and does not find use in controlling angular position. Prior art of this kind is disclosed in the specification of Japanese Patent Application Laid-Open No. 59-89820, by way of example.
An inductor motor having a magnetic bearing generally is not used in angular positioning, as mentioned above. If it is attempted to perform angular positioning by using such an induction motor, it is necessary to additionally provide a sensor of some kind for sensing the angle of rotation because the motor itself does not possess an angular positioning function. Accordingly, angular positioning must be carried out based on the output of the sensor. An example of such prior art is given in the specification of U.S. Pat. No. 4,180,946.